1. Technical Field
The invention relates to motor vehicle suspension systems and more particularly to a suspension system for motor vehicles which carry and support positionable loads such as aerial lift units.
2. Description of the Problem
Aerial lift units, used for accessing equipment mounted on utility poles or for tree trimming, are conventionally mounted on medium duty trucks. The truck provides a base for the aerial lift unit, and is subjected to differing rotational moments as the aerial lift unit is extended and positioned. Off center loads on the vehicle suspension obviously tend to cause the vehicle to tip to one side or to lean forward or backwards. To stabilize vehicles either outriggers have been used or the vehicles have been equipped with torsion bars. Outriggers are in effect struts which extend outwardly from the vehicle into contact with the ground to stabilize the vehicle. Outriggers though require side to side clearance which may or may not be available. The general alternative is to stiffen the vehicle's suspension with torsion bars. The torsion bars are oversized and vehicles so equipped require large amounts of ballast (up to 4500 lbs) to stabilize the chassis and allow movement of the aerial lift. Torsion bar stabilizing systems are very stiff. Some newer torsion bar designs use both front and rear stabilizing bars. With such suspension systems ride and comfort of the driver are sacrificed in the service of providing a base for the aerial lift unit. Torsion bar systems are preferred many times because the vehicles must operate in tight locations that don't allow outriggers to be deployed. A second advantage of a torsion bar system is the operators do not have to worry about putting the outriggers down before using the aerial device. Torsion bars also tend to lift wheels on the opposite side of the vehicle from the direction of deployment of the aerial lift. This is, up to a point, an advantage. With the wheels lifted, the spring suspension for those wheels does not continue pushing the vehicle in the direction in which the vehicle is leaning. While torsion bar systems offer some unique design solutions they bring with them some real trade-offs. A torsion bar chassis may be unable to level the chassis at the job site; ANSI regulations require that the chassis be level to within 5 degrees before use. This means that in some situations, when the operator cannot position the chassis within the 5-degree allowance, he must call in an aerial device with outriggers or perform the work in an unsafe condition.
Contemporary vehicles used to support aerial lift units that provide stabilization without outriggers use torsion bar systems. The ballast required by these systems is heavy and the torsion bars are expensive and dramatically affect the drivability of the vehicle. The torsion bars are up to three inches in diameter and can be difficult to install. The large quantities of ballast required dramatically reduce the carrying capacity and fuel economy of the chassis.